1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read only transponders, that is, adapted to operate with a terminal only reading the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and also on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal.
FIG. 1 very schematically shows, in a simplified way, a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10.
Generally, terminal 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3 (generally, the ground). The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and exploiting received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. In the example shown in FIG. 1, node 5 of connection of capacitor C1 with inductance L1 forms a terminal for sampling a data signal received from transponder 10 for the demodulator. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, essentially includes an inductance L2, in parallel with a capacitor C2 between two input terminals 11, 12 of a circuit 13 of control and processing of transponder 10. Terminals 11, 12 are, in practice, connected to the input of a rectifying means (not shown), the outputs of which define D.C. supply terminals of the circuits internal to the transponder. In FIG. 1, the load formed of the circuits of transponder 10 on the oscillating circuit have been modeled by a resistor R2, shown in dotted lines, in parallel with inductance L2 and capacitor C2.
The oscillating circuit of terminal 1 is excited by a high-frequency signal (for example, 13.56 MHz) intended for being sensed by a transponder 10. When transponder 10 is in the field of terminal 1, a high-frequency voltage is generated across terminals 11, 12 of the transponder's resonant circuit. This voltage, after being rectified, is intended for providing the supply voltage of electronic circuits 13 of the transponder. These circuits generally essentially include a microprocessor, a memory, a demodulator of the signals possibly received from terminal 1, and a modulator for transmitting information to the terminal.
The data transmission from transponder 10 to terminal 1 is generally performed by modifying the load of oscillating circuit L2, C2, so that the transponder draws a lesser or greater amount of power from the high-frequency magnetic field. This variation is detected, on the side of terminal 1, because the amplitude of the high-frequency excitation signal is maintained constant. Accordingly, a power variation of the transponder translates as a variation of the current amplitude and phase in antenna L1. This variation is then detected, for example, by a measuring the signal of terminal 5, either by means of a phase demodulator, or by means of an amplitude demodulator. The load variation on the transponder side is generally performed by means of an electronic switch for controlling a resistor or a capacitor modifying the load of the oscillating circuit. The electronic switch is generally controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1.
In the case of a phase demodulation by terminal 1, its modulator detects, in the sub-carrier half-periods when the electronic switch of the transponder is closed, a slight phase shift (by a few degrees, or even less than one degree) of the high-frequency carrier with respect to a reference signal. The demodulator output then provides a signal that is an image of the control signal of the electronic switch of the transponder, which can be decoded to restore the transmitted binary data.
To obtain a proper operation of the system, the oscillating circuits of terminal 1 and of transponder 10 are generally tuned on the carrier frequency, that is, their resonance frequency is set, for example, to the 13.56-MHz frequency. This tuning aims at maximizing the power transfer to the transponder, generally, a card of credit card size integrating the different transponder components.
The fields of application of electromagnetic transponders (for example, the crossing of highway tolls, the counting or the authentication of transponder carriers, etc.) may make it desirable to guarantee that a transponder only operates in a predetermined distance relation with a read/write terminal, more specifically in a distant relation, generally defined by a distance greater than 5 cm separating the respective antennas of the transponder and of the read/write terminal.
For example, in applications such as the crossing of highway tolls where the driver is necessarily far from the terminal, it is indispensable to guarantee the transaction security and prevent the pay or authentication signal from being intercepted by a “pirate” terminal placed closer to the transponder. In this case, it must be guaranteed that the transponder will only operate in a distant relation with the terminal.
Still as an example, when a transponder is in the terminal field, another transponder can also be in this field. Conventional systems then favor the operation of the transponder that is closest to the terminal. In some applications, it may however be desired to favor the operation of the most distant transponder. In this case, conventional systems provide no acceptable solution.
Indeed, a transponder that would then be closer to the terminal could capture the information transmitted by the terminal to the distant transponder, which does not provide the desired guarantees in terms of security. Further, a pirate terminal can then be interposed between the authorized terminal and the transponder and then intercept the transponder's information, which is not desirable either.
Another problem that is raised is that, in conventional systems, a transponder in a relatively distant coupling relation with a terminal will receive less power than a transponder placed closer to the terminal. In such a case, the system operation risks suffering therefrom.